The U.S. sees "strong indications" that Syria's government used chemical weapons in attacks early Wednesday that opposition groups claimed killed more than 1,100 people—in what would be a significant escalation of poison-gas use in the civil war.

Syrian authorities denied using chemical weapons in their renewed offensive on Wednesday, accusing the opposition of fabricating claims or staging gas attacks themselves. "These claims are categorically false and completely baseless and are part of the filthy media...